1. Field of the Invention
The present invention relates to block copolymers that can be formed into ion conductive membranes for fuel cell applications.
2. Background Art
Fuel cells are used as an electrical power source in many applications. In particular, fuel cells are proposed for use in automobiles to replace internal combustion engines. In proton exchange membrane (“PEM”) type fuel cells, hydrogen is supplied to the anode of the fuel cell and oxygen is supplied as the oxidant to the cathode. The oxygen can be either in a pure form (O2) or air (a mixture of O2 and N2). PEM fuel cells typically have a membrane electrode assembly (“MEA”) in which a solid polymer membrane has an anode catalyst on one face and a cathode catalyst on the opposite face. The MEA, in turn, is sandwiched between a pair of non-porous, electrically conductive elements or plates which (1) serve as current collectors for the anode and cathode, and (2) contain appropriate channels and/or openings formed therein for distributing the fuel cell's gaseous reactants over the surfaces of the respective anode and cathode catalysts.
In order to efficiently produce electricity, the polymer electrolyte membrane of a PEM fuel cell typically, must be thin, chemically stable, proton transmissive, non-electrically conductive, and gas impermeable. Moreover, during operation of the fuel cell, the PEM is exposed to rather severe conditions, which include, hydrolysis, oxidation and reduction (hydrogenation) that can lead to degradation of the polymer thereby reducing the lifetime of a polymer electrolyte membrane. The combination of these requirements imposes rather strict limitations on material choices for these membranes. Presently, there are relatively few polymer systems that provide even marginally acceptable results for the combination of these requirements. An example of a PEM is the Nafion membrane developed by DuPont in 1966 as a proton conductive membrane. This membrane is possibly the only advanced polymer electrolyte currently available for use in a membrane electrode assembly in a fuel cell.
Other polymer systems that may be used in PEM applications are found in U.S. Pat. No. 4,625,000 (the '000 patent), U.S. Pat. No. 6,090,895 (the '895 patent), and EP Patent No. 1,113, 517 A2 (the '517 patent). The '000 discloses a sulfonation procedure forming poly(ether sulfone)s that may be used in solid polymer electrolyte application. However, the '000 patent's post-sulfonation of preformed polymers offers little control of the position, number, and distribution of the sulfonic acid groups along the polymer backbone. Moreover, the water uptake of membranes prepared from post sulfonated polymers increases, leading to large dimensional changes as well as a reduction in strength as the degree of sulfonation increases.
The '895 patent discloses a process for making cross linked acidic polymers of sulfonated poly(ether ketone)s, sulfonated poly(ether sulfone)s, sulfonated polystyrenes, and other acidic polymers by cross linking with a species which generates an acidic functionality. However, this reference does not suggest an effective way to cast membranes from those cross linked sulfo-pendent aromatic polyethers.
The '517 patent discloses a polymer electrolyte containing a block copolymer comprising blocks having sulfonic acid groups and blocks having no sulfonic acid groups formed by post sulfonation of precursor block copolymers consisting of aliphatic and aromatic blocks. In this patent, the precursor block copolymers are sulfonated using concentrated sulfuric acid, which leads to the sulfonation of aromatic blocks. However, once again, this post sulfonation of aromatic blocks offers the little control of the position, number, and distribution of the sulfonic acid groups along the polymer backbone. Furthermore, this post sulfonation of precursor block copolymers also leads to the cleavage of chemical bonds of the aliphatic block.
Although some of the proton conducting membranes of the prior art function adequately in hydrogen fuel cells, these membranes tend to require high humidity (up to 100% relative humidity) for efficient long-term operation. Moreover, prior art membranes are not able to efficiently operate at temperatures above 80° C. for extended periods of time. This temperature limitation necessitates that these membranes be constantly cooled and that the fuel (i.e., hydrogen) and oxidant be humidified.
Accordingly, there exists a need for improved materials for forming polymer electrolyte membranes and for methods of forming such materials.